1. Field of the Invention
This invention relates to methods and apparatus for gravel packing well formations, and more particularly to a gravel packing method and apparatus wherein downhole mixing of a gel slurry with a breaker fluid is utilized.
2. Description of the Prior Art
Unconsolidated formations, particularly those containing loose sands and soft sandstone strata, present constant problems in well production due to migration of loose sands and degraded sandstone into the well bore as the formation deteriorates under the pressure and flow of fluids therethrough. This migration of particles may eventually clog the flow passages in the production system of the well, and can seriously erode the equipment. In some instances, the clogging of the production system may lead to a complete cessation of flow, or "killing" of the well.
One leading method of controlling sand migration into a well bore consists of placing a pack of gravel on the exterior of a liner assembly having a perforated or slotted liner or screen which is positioned across an unconsolidated formation, commonly referred to as the "zone" to be packed. The gravel pack presents a barrier to the migrating sand from that formation while still permitting fluid flow. The gravel is carried to the formation in the form of a slurry. The proper size of gravel must be employed to effectively halt sand migration through the pack, the apertures of the liner or screen being adapted so that the gravel will not pass through it.
The gravel is a solid, particulate material, such as very fine sand.
Once the liner or screen is positioned across the zone, a packer is set above the zone between the liner and the well casing, or, if unlined, the well bore wall, to isolate that zone from those above. The zone may be at any point in the well.
The solid material used in the gravel packing is pumped down the tool string in a gravel slurry formed by mixing the solids in a carrier fluid. The slurry passes through a crossover tool and out into the annulus between the liner and the casing at a suitable location above the zone, and the gravel is deposited in the area around the screen.
Typical apparatus and methods for gravel packing are disclosed in U.S. Pat. Nos. 4,270,608; 4,273,190; 4,295,524; 4,296,807; and 4,418,754.
For the purposes of distributing the gravel into the formation, a low viscosity gravel slurry, such as an aqueous solution, is preferable, because the low viscosity fluid will flow well into the formation, and thus carry the gravel further into the formation for better packing. This reduces the possibilities of washing out the gravel when production fluids are flowed therethrough.
However, a low viscosity carrier fluid is frequently not adequate for several reasons. First of all, a low viscosity fluid cannot suspend or hold as much gravel therein as a higher viscosity fluid. Further, with low viscosity fluids, the gravel tends to settle out into the tubing string before it reaches the well annulus adjacent the formation. To solve this "sand-out" problem, higher viscosity gel slurries have been developed. Generally, the carrier fluid in such a slurry is formed by mixing an aqueous or petroleum base solution with a gelling agent.
Such gelled slurries perform well in carrying the sand down the tubing string into the well annulus, but because of the high viscosity of the carrier fluid, the sand is not distributed as well into the formation. This results in poorer pack density and increased chances of premature erosion of the formation. Further, high viscosity fluids are more likely to result in formation damage.
An additional problem with gelled slurries is that it takes longer for the sand to settle out once it is in the annulus. This increases the waiting time at the surface and therefore costs are higher.
Thus, it has been recognized in the sand control field that the ideal gravel pack method would be to use a gel as a carrier fluid down to the top of the screen, then use a low viscosity carrier fluid, such as an aqueous solution, throughout the rest of the gravel packing job. This method would present the best advantages of both a gelled carrier fluid and a low viscosity carrier fluid. As a result, methods have been developed to break the gel down when it reaches the desired location above the liner screen. Such methods include employing a gel based carrier which will automatically break down after a certain period of time. Such a method is described in U.S. Pat. No. 4,259,205. A problem with such methods is that once the slurry has been mixed, the operation must be carried out within the predetermined time. If there are any delays in the gravel packing operation, the gel slurry will break down prematurely, such as when it is being pumped down the tubing string. This has the undesirable result of the sand settling out prematurely or "sanding-out", as is the case with aqueous carrier fluids.
The method of the present invention solves the problems with previous gravel packing operations in that a gel slurry is used to carry the sand down the hole and is mixed at a predetermined location above the screen with a breaker fluid in place in the well annulus. A mixing valve positioned above the packer is utilized for this purpose.
A mixing valve for use in cementing, fracturing and other treatment of wells, is shown in U.S. Pat. No. 4,361,187, assigned to the assignee of the present application and incorporated herein by reference.